This invention relates generally to xerographic copying apparatus and, more particularly, to a contact fusing system for fixing electroscopic toner material to a support member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual image can be either fixed directly upon the photosensitive member or transferred from the member to a sheet of plain paper with subsequent affixing of the image thereto.
In order to permanently affix or fuse electroscopic toner material onto a support member by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to be absorbed to some extent into the fibers of the support member which, in many instances, constitutes plain paper. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be firmly bonded to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy for fixing toner images onto a support member is old and well-known.
One approach to thermal fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is either externally or internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll to thereby effect heating of the toner images within the nip. By controlling the heat transfer to the toner, virtually no offset of the toner particles from the copy sheet to fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the hot offset temperature of the toner whereat the toner particles in the image areas of the toner would liquefy and cause a shearing action in the molten toner to thereby result in "hot offset". Shearing occurs when the inter-particle forces holding the viscous toner mass together is less than the surface energy forces tending to offset it to a contacting surface such as the fuser roll. Further, by the provision of the proper roll surface material, therefore, abhesive materials, offset of toner particles is minimized.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e., "cold offsetting") by imperfections in the properties of the entire surface of the roll; or by the toner particles insufficiently adhereing to the copy sheet by the electrostatic forces which normally hold them there. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip and before the backup roll can be moved out of contact with the fuser roll.
Moreover, toner particles can be picked up by the fuser and/or backup roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus.
It will be appreciated that in order to prevent such toner particles being transferred to the copy paper, it is necessary to remove the toner particles from the fuser roll and/or the backup roll. It will be further appreciated that if enough toner accumulates on the fuser and/or backup roll, the paper feed through the fusing system will be adversely affected.
One arrangement for minimizing the foregoing problems, including that which is commonly referred to as "offsetting", has been to provide a fuser roll with an outer surface or covering of polytetrafluoroethylene commonly known as Teflon, to which a release agent such as silicone oil is applied. Silicone based oils, which possess a relatively low surface energy, have been found to be a material that is suitable for use in the heated roll fuser environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to thereby form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from adhering or offsetting to the fuser roll surface.
Another arrangement for minimizing the aforementioned "offsetting" problem is to provide a fuser member, for example, a fuser roll or belt structure with an outer surface or layer of silicone rubber which due to its elastic properties during operation, prevents toner offset thereto. The foregoing is disclosed in U.S. Pat. No. 3,666,247 issued May 30, 1972 wherein it is stated that "while the toner is still in a molten state and when the deformed elastomer is in the process of returning to its original shape, it separates from the toner thereby setting up a shear force which prevents the toner from offsetting to the heated element".
While it may be desirable to completely avoid the employment of release material from the standpoint of overall cost of operation of the apparatus, utilizing a contact fuser having a conformable surface such as silicone rubber which has a continuous supply of silicone oil provided therefor has been found to yield superior results in preventing toner offsetting accompanied by a considerable cost savings and very wide latitude in operation. Systems are known which apply silicone oil to a silicone rubber surface wherein i.e., U.S. Pats. Nos. 3,716,221 and 3,731,358, however, only in an intermittent fashion which is adequate for low volume and light density image applications. Low volume refers to fewer images rather than fewer copies.
While U.S. Pat. No. 3,731,358 may be considered for its disclosed application as being a continuous silicone oil system it has been determined that the capacity of such a system is quite limited. Such limitation being attributable to the relative restrictions imposed upon the thickness of the silicone rubber employed as the outer surface of that fuser construction due to the fact that silicone rubber severely retards the flow of thermal energy from within the fuser roll structure to without.
Accordingly, it is a primary object of this invention to provide a new and improved fuser apparatus.
Another object of this invention is to provide an improved fuser apparatus wherein a release material is continuously applied to the surface of a fuser roll structure having an elastomeric surface wherein the release material is impregnated in the elastomeric surface.
Still another object of this invention is to provide a silicone rubber surfaced fuser roll structure having increased capacity of release agent material for coating the surface thereof to prevent offset of toner and to facilitate stripping of copy paper therefrom.
Other objects and advantages of the invention will become apparent from the detailed description of the invention when read in conjunction with the accompanying drawings.